Electrophotographic printers with detachably mountable developing devices are well known in the art. The developing device is provided with an outer case rotatably supporting a developing roller that serves to carry toner on the surface thereof; a thickness-regulating blade regulating a thickness of a layer of toner carried on the developing roller; and side seal members designed to prevent toner from leaking out of axial ends of the outer case with respect to an axial direction of the developing roller.
One such developing device that has been proposed is a developing cartridge provided with side seal members, each configured of an upstream seal member and a downstream seal member. The upstream seal member is disposed upstream of the downstream seal member with respect to a rotating direction of the developing roller. The upstream seal members are respectively disposed between left and right sides of the outer casing and corresponding axial ends of the developing roller and are fixed to the outer case. The downstream seal members are respectively disposed between left and right ends of the thickness-regulating blade and corresponding axial ends of the developing roller and fixed to both the respective upstream seal members and the thickness-regulating blade.
In the conventional developer cartridge described above, the upstream and downstream seal members are disposed adjacent to each other in the rotating direction of the developing roller. The upstream seal member restricts leakage of toner between the respective left or right end of the outer case and the corresponding axial end of the developing roller, while the downstream seal member restricts the leakage of toner through the respective left or right end of the thickness-regulating blade and the corresponding axial end of the developing roller.